1. Field of the Invention
The present invention relates to improvements in methods and apparatus for aspirating a predetermined volume of liquid from a container (e.g., a test tube or vial) with an aspirating probe or needle. More particularly, it relates to improvements in methods and apparatus for assuring that the aspirating probe is safely positioned within the liquid sample during the entire liquid-aspirating process to avoid an unintentional introduction of air into the aspirated volume. The invention is particularly useful in the fields of hematology, fluorescence flow-cytometry and blood chemistry where it is often necessary to aspirate and dispense, with high precision, relatively minute volumes (of the order of microliters) of blood and liquid reagents used for the analysis of such blood.
2. Discussion of the Prior Art
Automated hematology instruments typically include apparatus for automatically aspirating a blood sample from a sealed test tube and for dispensing one or more precise aliquots of the aspirated sample to a workstation for processing. In such instruments, a hollow sample-aspirating needle is automatically advanced downwardly, usually along a vertical path coincident with the longitudinal axis of the blood-containing test tube. During such movement, the sharp distal end or tip of the aspirating needle punctures the septum that seals the tube, travels through a volume of air positioned between the sample and the septum, and eventually enters into the blood sample to be aspirated. Thereafter, a vacuum pump is activated for a predetermined time interval to aspirate a desired volume (e.g., 250 microliters) of sample into and through an internal lumen of the aspirating needle to which the vacuum pump is operatively connected by a suitable conduit. The aspirated sample is typically segmented (e.g., by a conventional blood-sampling valve of the type disclosed in the commonly owned U.S. Pat. No. 4,896,546 to Cabrera et al.) to provide a plurality of relatively small aliquots (each being between about 5 and 75 microliters) which are then dispensed and mixed with suitable reagents in which the sample aliquots are analyzed. Alternatively, the aspirated sample can also be segmented by means of a positive-displacement (syringe-type) pump in a system in which the aspirating probe also functions as a dispensing probe.
In blood-aspirating apparatus of the above type, it will be appreciated that the preciseness of the sample volume aspirated requires that the probe tip be completely submerged in the blood sample at all times during the aspiration process. Should the aspirating vacuum force be applied to the probe for a period of time while the probe tip is positioned outside the sample volume, e.g., within the air pocket above the sample, air will be drawn into the probe, and the precision of the aspiration will be compromised. This condition is exacerbated in a sample aspirate/dispense system of the above-mentioned type since such a “suck and spit” system does lend itself to the use of an in-line bubble detector. Various schemes have been proposed and used to date for avoiding the aspiration of air into the sample line. For example:
In the commonly owned U.S. Pat. No. 4,341,736 to Drbal et al., a fluid (i.e., liquid) transfer mechanism is disclosed for aspirating biological fluid samples from a series of open cuvettes or containers. This patent discloses two different schemes for signaling that the tip of a fluid-aspirating probe is safely submerged within a fluid sample so that aspiration can occur without drawing air into the aspirated sample. Both schemes make use of the electrical properties of the fluid sample in generating the signal. According to a first scheme, the probe is constructed from a non-conductive plastic tube. The tube supports a pair of spaced, parallel electrodes that run along the entire tube interior and terminate at the aspirating end of the tube. The opposite ends of the electrodes are connected across an electrical power supply. As the electrode ends on the probe tip move towards and eventually contact the sample fluid, an electrical circuit is completed through the sample fluid; thus, a signal is produced indicating that the probe tip has now entered the sample fluid and aspiration can safely occur. According to the second scheme, the aspirating probe is made of stainless steel, and the sample fluid is contained either in an electrically conductive container, or if the container is non-conductive, the container is supported on a conductive base. The steel probe is electrically connected to an AC power source, and the conductive container or base is electrically grounded. As the probe tip moves towards and eventually contacts the surface of the sample fluid, a change in electrical capacitance occurs between the probe tip and container (or base), as determined by the dielectric properties of the intervening sample fluid. This capacitance change is detected in a bridge circuit that signals that the probe has contacted and entered the sample fluid.
In using liquid level-sensing apparatus of the type noted above, a problem can arise when the liquid sample to be aspirated is subject to foaming when agitated. In the field of hematology, it is common to continuously rock, and thereby agitate, a blood sample in a test tube to assure, for example, the homogeneity of the sample during analysis. The continuous movement of the blood forward and backward in the vial can eventually lead to the formation of a bubbly mixture of air and blood (i.e., foam) on the surface of the sample. Unfortunately, the electrical properties of the foam differs little from the sample itself. Thus, in the above apparatus, as soon as the electrodes or conductive aspirating probe contacts the foam, a signal is generated indicating that the probe is in a position ready for sample aspiration. When this occurs, air bubbles can be drawn into the aspirated sample, thereby making the aspirated volume uncertain. Further, in liquid level sensors of the capacitance level-sensing type, it is usually necessary to maintain the test tube (when it is non-conductive) in a generally upright orientation at all times. This orientation assures that the counter electrode (the conductive base on which the test tube rests) is in close proximity to the sample liquid. Thus, this liquid level-sensing scheme is not useful in instruments in which the test-tube is inverted (with its seal facing downward) during sample aspiration.